The preparation of polyaspartic acid and salts thereof by acid-catalyzed thermal polycondensation of aspartic acid to polyaspartimide and subsequent alkaline hydrolysis of the polyaspartimide is known in principle. The acidic catalysts used are, for example, mineral acids such as phosphoric acid, phosphorous acid, sulfuric acid and sulfurous acid. It is also possible to use organic acids such as methanesulfonic acid or amidosulfonic acid. Phosphoric acid has proven to be particularly suitable as a moderately strong and non-oxidative acid.
Methanesulfonic acid (MSA) is also a suitable catalyst due to its non-oxidative effect.
Preferably, acids such as phosphoric acid serve not only as catalyst but also as solvent. The advantage of a well-controlled polycondensation, when phosphoric acid is catalyst and solvent at the same time, is set against the disadvantage of a necessary product purification. The acid has to be removed by washing and, for reasons of cost, should be laboriously recovered. The alternative would be the use of only small amounts of catalyst. If, however, only small amounts of the acidic catalyst (1 to 25 mol %, based on the amount of aspartic acid used) are used, this results during the condensation in highly viscous to very hard condensation phases which tend to become caked, which in stirring apparatuses or kneaders can no longer be stirred or kneaded. The consequence is that either the condensation has to be stopped or at least interrupted in order to break up again and to comminute baked solid polycondensate. Only then can the polycondensation be continued. For instance, U.S. Pat. No. 5,457,176 A describes the thermal polycondensation of aspartic acid using catalytic amounts of phosphoric acid or methanesulfonic acid. In both examples, the polycondensation is interrupted, the solid intermediate laboriously isolated and comminuted and the comminuted intermediate is fed back to the reaction vessel to complete the condensation. A similar process is described in DE 4023463 A1, where phosphoric acid is used as catalyst in the condensation of aspartic acid and the resulting reaction product has to be mechanically comminuted in a second stage.